libcpprestsdk: fix building as a static library

Use pkg-config to find OpenSSL. This will automatically find any
dependent libraries and put them in the correct order for linking.

Upstream status: submitted
https://github.com/Microsoft/cpprestsdk/pull/688

Fixes:

  http://autobuild.buildroot.net/results/be9e8d1717968a0ff8f01f7fadfa79825ac88b94/



Signed-off-by: Adam Duskett <aduskett@gmail.com>
Reviewed-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>